Various compensation techniques have been applied to active matrix organic light-emitting diode (AMOLED) pixel circuits in order to compensate for the non-uniformity of threshold voltage of the driving transistors due to the process.
FIG. 1 is a schematic diagram showing a compensation technique for an AMOLED pixel circuit. As shown in FIG. 1, the source of a driving transistor DT is connected to a first power supply voltage Vdd, an organic light-emitting diode (OLED) is connected between the drain of the driving transistor DT and a second power supply voltage Vss, and a compensation transistor CT is connected between the gate and the drain of the driving transistor DT. When the gate of the compensation transistor CT is applied with a low level signal SN, the compensation transistor CT is turned on to cause the driving transistor DT to form a diode structure. This diode structure can be used to compensate for the influence of the threshold voltage Vth of the driving transistor DT on the driving current IOLED supplied by the driving transistor DT to the OLED.
When the driving transistor DT forms the diode structure, its gate voltage Vg is equal to its drain voltage Vd, and the difference Vds between the drain voltage Vd and the source voltage Vs satisfies Vds=Vth. Accordingly, it can be derived in the example of FIG. 1 that Vg=Vd=Vds+Vs=Vth+Vdd. As can be seen, the threshold voltage Vth of the driving transistor DT has been introduced into the gate voltage Vg.
In a subsequent light emission phase, the drive current IOLED can be calculated as:IOLED=K(Vgs−Vth)2  (1)where K represents a constant value determined by the mobility of the driving transistor DT and the parasitic capacitance, and Vgs represents the difference between the gate voltage Vg and the source voltage Vs of the driving transistor DT. During this light emission phase, the compensation transistor CT is turned off and the data voltage has been introduced into Vg so that the threshold voltage Vth is canceled out in equation (1). In other words, the driving current IOLED will be independent of the threshold voltage Vth, thus improving the uniformity of luminance across individual OLEDs.
However, a leakage current flowing through the compensation transistor CT causes a change in the gate voltage Vg of the driving transistor DT when the compensation transistor CT is turned off. This may, for example, result in a change in the driving current IOLED during the light emission phase, thus affecting the display quality.